1. Field of the Invention
The present invention relates to an image display method.
2. Description of the Related Art
Image display devices are roughly classified into impulse type display devices such as CRTs and hold type display devices such as LCDs (Liquid Crystal Displays). Impulse type display devices display images only while a phosphor is emitting light after the images have been written thereto. Hold type display devices hold an image in the preceding frame until a new image is written thereto.
A problem with the hold type display is a blur phenomenon that may occur when motion pictures are displayed. The blur phenomenon occurs because if a person observes a moving object on a screen, his or her eyes continuously follows the moving object though an image in the preceding frame remains displayed at the same position until it is switched to an image in the next frame. That is, in spite of the discontinuous movement of the moving object displayed on the screen, the eyes perceive the moving object in such a manner as to interpolate an image between the preceding and next frames because the following movement of the eyes is continuous. As a result, the blur phenomenon occurs.
To solve such a problem, a display method based on a field inversion system has been proposed (Jpn. Pat. Appln. KOKAI Publication No. 2000-10076) which utilizes such an operational characteristic of a monostable liquid crystal that one polarity allows the transmittance of light to be controlled in an analog manner, whereas the other polarity prevents light from being transmitted. With this display method based on the field inversion system, one frame is divided into two subfields. One of the subfields allows a liquid crystal to transmit light therethrough, whereas the other prevents the liquid crystal from transmitting light therethrough. A display method using bend alignment cell has also been proposed (Jpn. Pat. Appln. KOKAI Publication No. 11-109921). Both proposals provide periods when original images are displayed and periods when black images are displayed to approximate the impulse type display.
However, with the method based on the field inversion system, a voltage must be applied to a positive and negative polarities for an equal period so that no DC components remain in a liquid crystal layer. Consequently, the display has a duty ratio of 50%. In this case, the following definition is given: “duty ratio=display period/(display period+non-display period)×100”.
With the method using bend alignment cell, to change the duty ratio, the number of dividing must be increased. Consequently, differences between signal line driving circuits make the display ununiform (a variation in brightness (i.e. luminance)). Further, a driving frequency for scanning lines must be changed in order to change the duty ratio. However, it is difficult to strictly set the duty ratio.
Furthermore, when the duty ratio is changed to increase the black display period, the brightness of the entire screen decreases. In this case, for a liquid crystal display device, the maximum brightness of a back light must be increased. However, this leads to an increase in power consumption. Moreover, if the duty ratio is varied by blinking the back light, flickers may occur unless the back light can blink stably.
Thus, with the conventional methods, providing black display periods may cause a decrease in screen brightness or the like. This may result in various problems.
On the other hand, color image display operations based on an additive color mixing system involve a spatial additive color mixing system and a field-sequentially additive color mixing system. With the spatial additive color mixing system, an R (Red) pixel, a G (Green) pixel, and a B (Blue) pixel which are adjacent to one another constitute one pixel so that the three-primary colors (R, G, and B) can be spatially mixed together. With the field-sequentially additive color mixing system, an R, G, and B images are sequentially displayed so that the three-primary colors can be mixed together in the direction of a time base. With this system, the R, G, and B images are mixed together at the same location. Consequently, it is possible to increase the resolution of the color image display device.
Field sequential color display operations utilizing the field-sequentially additive color mixing system involve various systems such as a color shutter system and a three-primary-color back light system. With any of these systems, an input image signal is divided into an R, G, and B signals. Then, the corresponding R, G, and B images are sequentially displayed within one frame period to achieve color display. That is, with a field sequential color display device, one frame is composed of a plurality of subfields that display R, G, and B images.
In general, a display device requires that one frame frequency is equal to or larger than a critical fusion frequency (CFF) at which no flickers are perceived. Accordingly, with the field sequential color display, when the number of subfields within one frame is defined as n, each subfield image must be displayed at a frequency n times as high as a frame frequency. For example, as shown in FIG. 24, if one frame frequency is 60 Hz and three subfields for R, G, and B are used to perform a field sequential color display operation, each subfield has a frequency of 180 Hz.
Methods for implementing a field sequential color display operation include the temporal switching of an RGB filter and the temporal switching of an RGB light source. Examples of the use of the RGB filter include a method of using a white light source to illuminate a light bulb and mechanically rotating an RGB color wheel and a method of displaying black and white images on a monochromatic CRT and providing a liquid crystal color shutter on a front surface of the CRT. An example of the use of the RGB light source is a method of illuminating a light bulb using an RGB LED or fluorescent lamp.
The field sequential color display operation must be performed at high speed. Accordingly, a light bulb for displaying images is composed of a quickly responsive DMD (Digital Micromirror Device), a bend alignment liquid crystal cell (including a PI twist cell and an OCB (Optically Compensated Birefringence) mode with phase compensating films added thereto), a ferroelectric liquid crystal cell using a smectic liquid crystal, an antiferroelectric liquid crystal cell, or a V-shaped responsive liquid crystal cell (TLAF (Thresholdless Anti-Ferroelectric) mode) exhibiting a voltage-transmittance curve indicative of a thresholdless V-shaped response. The light bulb may also be used for a liquid crystal cell used in a liquid crystal color shutter.
As described previously, in the field sequential color display operation, the lower limit on the subfield frequency at which no flickers are perceived is 3×CFF, i.e. about 150 Hz. It is known that a low subfield frequency may lead to “color breakup”. This phenomenon occurs because an R, G, and B images do not coincide with one another on the retina owing to movement of the eyes following motion pictures or for another reason, thereby making the contour of the resulting image or screen appear colored.
For example, if an image signal for one frame has a frequency of 60 Hz, an R, G, and B subfield images are each displayed all over a display screen at a frequency of 180 Hz. If an observer is viewing a still image, an R, G, and B subfield images are mixed together on the observer's retina at a frequency of 180 Hz. The observer can thus view the correct color display. For example, when an image of a white box is displayed in the display screen, an R, G, and B subfields are mixed together on the observer's retina to present the correct color display to the observer.
However, if the observer's eyes move across the displayed image in the direction shown by the arrow in FIG. 23A, then as shown in FIG. 23B, at a certain instant, an R subfield image is presented to the observer, at the next instant, a G subfield image is presented to the observer, and at the next instant, a B subfield image is presented to the observer. Since the observer's eyes are moving across the display screen, the R, G, and B images do not perfectly coincide with one another on the observer's retina. Instead, the images are mixed together in such a manner as to deviate from one another. Thus, in the vicinity of an edge of a moving object, an R, G, and B subfields are not mixed together but individually appear. As a result, color breakup may occur. This is due to jumping movement of the eyes. Further, although the observer's eyes follow the moving object, each subfield image is displayed at the same location for one frame period. Accordingly, on the observer's retina, subfield images are mixed together in such a manner as to deviate from one another. As a result, the hold effect of the eyes may cause similar color breakup. Such a phenomenon strikes the observer as incongruous. Further, if the display device is used for a long time, the observer may be fatigued.
The color breakup caused by the jumping movement of the eyes can be suppressed by increasing the subfield frequency. However, this method fails to sufficiently suppress the color break up resulting from the hold effect. The color breakup resulting from the hold effect can be reduced by substantially increasing the subfield frequency. However, substantially increasing the subfield frequency creates a new problem. That is, loads on driving circuits for the display device may increase.
As described above, in the methods proposed to prevent motion pictures from blurring, one frame is divided into subfields used for image display and subfields used for black display. However, disadvantageously, the brightness of the image may generally decrease or the maximum brightness of the image must be increased. As a result, it is difficult to obtain high-quality images.
Further, if color images are displayed on the basis of the field-sequentially additive color mixing system by dividing one frame into a plurality of subfields, then possible color breakup makes it difficult to obtain high-quality images. Further, if the subfield frequency is increased to suppress the color breakup, loads on the driving circuits may disadvantageously increase.
It is an object of the present invention to provide an image display method that provides high-quality motion pictures.